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Old 01-10-2013, 12:35 PM
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1157 bulbs

Can someone explain to me how a dual contact bulb, such as an 1157, works? I understand the brass base is ground and the two contacts are switched hot, at least I think that's true based on my limited knowledge. When I check continuity between one of the two contacts and the brass base, it reads an open circuit. When I check continuity between the two contact points, I get a closed circuit reading. Neither reading makes sense to me.

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Old 01-10-2013, 12:41 PM
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Best way I can explaine it.. It's really two bulb's in one..
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Old 01-10-2013, 12:49 PM
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1157 bulbs

I get the notion that it's two bulbs in one. Assuming that's true, why aren't the elements completely separate? Why would there be a closed circuit between them? Shouldn't that be open?
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Old 01-10-2013, 01:07 PM
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bulb

they share a common ground.. the copper base. so measuring between the 2 buttons will have a small resistance.. but power will only flow from one button to ground..
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Old 01-10-2013, 01:15 PM
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Quote:
Originally Posted by jax32 View Post
I get the notion that it's two bulbs in one. Assuming that's true, why aren't the elements completely separate? Why would there be a closed circuit between them? Shouldn't that be open?
They are different.. One will light for running lights, And the other will light for stop or turn... They can't be connected are they wouldn't do two different things..
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Old 01-10-2013, 01:22 PM
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" When I check continuity between one of the two contacts and the brass base, it reads an open circuit."
Usually this happens with a burned out filament. If the bulb is known good, you are just not making contact with one or both probes from the continuity checker. Clean the contacts, and the base of the bulb until shiny, and try again.

When I check continuity between the two contact points, I get a closed circuit reading."
This is normal with a good bulb. The electricity is going in to one contact, through one filament, into the base, that has a connection to both filaments, and through the second filament, and out the second contact.
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Old 01-10-2013, 01:37 PM
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1157 bulbs

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This is normal with a good bulb. The electricity is going in to one contact, through one filament, into the base, that has a connection to both filaments, and through the second filament, and out the second contact.
This is the part I don't understand. I've always viewed dual-filament bulbs as two bulbs in one that happened to share a common base or ground. In other words, it would be like two single-filament bulbs side by side. In dual element bulbs, I don't understand why, as an example, when the parking lights are turned on the turn signal filament doesn't glow, and vice versa.
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Old 01-10-2013, 03:03 PM
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Sir, daniel just told you , 1 base lug thru filament thru ground [brass body] thru other filament to other base lug !!

dave
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Old 01-10-2013, 03:22 PM
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Lets start with this, a dual filament headlight plug.

This lamp is wired just like a 1157 bulb, only it is bigger.

The ground terminal is connected to the negative battery terminal, just like the brass base on a 1157 bulb.
When the low beam headlights are on, the electricity goes from the low beam terminal, through the low beam filament, to ground. The electricity cannot go back out the high beam terminal, because it is not connected to anything. In the same manner, when the tail lights are on, the electricity goes from the tail light contact on the base of the lamp, to the tail light filament, to the base, and ground.

When you turn on a turn signal, the turn signal switch also applies power to the turn signal contact on the 1157 lamp, goes through the "bright" filament, and then to the brass base of the bulb, and then ground.

Think of it this way. You could have completely separate lights for turn, and tail lights, on each side of the car. You could a wire for the tail light switch, to the tail light, and a ground wire all the way back to the battery. Then you could do the same thing for the turn signal switch, the turn signal lamp, and a separate ground wire back to the battery.
You now have two ground wires at the battery. One wire will do. Why not just connect the turn signal ground wire, and the tail light ground to one wire, with a "Y" connection, and run only one ground wire to the battery?

The 1157 lamp does exactly that, but the ground wire "Y" is inside the lamp, so you only need one ground wire for the two lamps, in one bulb.
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Old 01-10-2013, 03:23 PM
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Dave, Not to belabor the point but I understand WHAT happens. I'm trying to understand WHY it happens, especially since they appear to be two separate circuits.
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Old 01-10-2013, 03:48 PM
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This is why.
Electricity follows the path of least resistance. Once the electricity has gone through the tail light filament, and to the base of the lamp, it is easier for the electricity to go to the battery negative terminal, ground, than it is to go back and do more work lighting the other filament.
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Old 01-10-2013, 04:06 PM
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You might say that "work", aka resistance, is something that only happens at the point between where pos and negative meet each other. Both filaments are grounded but only the one that has positive meeting it from the other side will light. The other one just sits there.

By-the-way you could apply positive to one center contact and negative to the other, and both filaments would light, connecting through the ground in the base...if they were the same size. Because they are different, only the smaller one will light, the other doesn't get hot enough.

And I wouldn't know why you don't get continuity between the base and center contacts. You should.
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Old 01-10-2013, 04:15 PM
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Re-wording what was said above
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